In any optical data storage device, be it a video disk, a compact disk player, a compact disk ROM data reader or a machine capable of reading and writing, the focused point of light must be made to follow the data track with high accuracy. This involves automatically maintaining focus and tracking to fractions of a micron. The actuators commonly used to do this are large and heavy, compared to the actual optics forming this focused point. This size and weight reduces the speed with which such a device can be made to search the data region, thus limiting the applicability of these devices to a small fraction of the data storage market.
The conventional method for causing the focused spot to follow the track has been by moving the focussing objective lens. Such a device first moves the entire optics to a position above the general region of the data to be accessed. The lens is then moved axially, that is, parallel to the rotating axis of the disk, to maintain the accuracy of focus while the disk is spinning. The lens is also moved from side to side to follow the track, which is never perfectly circular. This means that the objective lens is housed in a device, itself movable and capable of moving the objective lens in two directions, and containing the actuating motors for those motions. These motors, while already quite miniaturized, are bulky and massive compared to the lens itself. The faster the disk spins, which is a desirable attribute from a system performance standpoint, the harder this motor must work and the bulkier and heavier it must become to provide accurate tracking and focus. Also, because the objective lens is not hard-mounted, but supported on flexible mounts, it will always be less rugged than it could be if fixed in place. The construction limit the speeds at which the objective lens can safely be moved and how forcefully it can be actuated.
Other systems described in the literature have improved on construction by splitting the optics into fixed and moving sections. This is described in a published paper: Magneto-Optical Disk For Coded Data Storage, M. Ojima, et. al. SPIE Vol. 529, Optical Mass Data Storage (1985) p. 12. In the system described in the article, only the focusing objective lens and its two dimensional actuator are moved over the disk, and the stated access time of 100 msec is fast for a read/write device. However, the requirement to move the actuator over the disk for scanning is still one of the characteristics limiting this speed.
It has been demonstrated that one of these functions, the tracking action, can be accomplished without moving the objective lens from side to side. A mirror mounted on a galvanometer can steer the beam of light, and if the system is properly designed, this beam will still enter the objective and be focussed on the disk. The spot is moved from side to side without moving the objective lens. However, this lens must still be actuated axially to maintain focus.
The optical system in accordance with the invention discloses a construction that, by reducing mass, complexity, and delicacy of the optical system or subsystem to be moved for scanning the disk, allows faster access from one section of a data storage disk to another. Such an improvement in speed allows use of optical storage devices in more demanding applications, where Winchester drives are now used in small, medium and large computer systems.
A movable optical head structure, that is, the structure movable over the disk surface to scan the disk, includes a fixed optical arrangement. Fine focussing and side-to-side adjustments for scanning are effected by movements of another lens mounted on a substantially stationary part of the system structure. This movable lens is generally aligned with the axis of the fixed lens on the movable head structure. Thus, motors for adjusting the optical system are not present on the movable head structure, which accordingly can be light in weight although ruggedly constructed. In an alternative embodiment, lateral adjustments to the position of the scanning spot are made by pivoting a reflecting mirror on the stationary structure and axial or focussing adjustments of the scanning spot are made by axially moving the aforesaid lens on the fixed structure. Light paths on the movable head structure adjacent the recording disk may be altered by means of reflecting mirrors, all such mirrors being in fixed relationship with the fixed objective lens. The movable head structure bearing fixed lenses and fixed mirrors moves in an arc to traverse the tracks on the disk in order to locate the desired track. The center of arc of the head structure is not the center of disk rotation.
Accordingly, it is an object of this invention to provide an improved optical system for fast access optical storage which allows for rapid physical response in focussing on a disk track, which may not be concentric to the axis of rotation, nor be on a flat surface.
A further object of this invention is to provide an improved optical system for fast access optical storage which positions drive and focus motors on stationary portions of the scanning structure thereby allowing rapid response time of the moving portions.
A further object of this invention is to provide an improved optical system designed for fast access optical storage which uses a rigidized optical subsystem on the moving head portion of the scanning apparatus, providing a rugged construction.
Still other objects and advantages of the invention will in part be obvious and in part be apparent from the specification.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.